1. Field of the Invention
The present invention relates in general to transmission lines for conveying high frequency signals, and in particular to a method and apparatus for adjusting the signal path delay of a transmission line.
2. Description of Related Art
Controlled impedance transmission lines are often used to convey high frequency digital signals between signal transmitters and receivers within an electronic circuit. The “characteristic impedance” Z0 of a uniform transmission line is often modeled asZ0=(L/C)1/2  [1]where L and C are the series inductance and shunt capacitance per unit length of the transmission line. Since abrupt changes in the characteristic impedance of a transmission line can cause undesirable signal reflections, circuit designers usually try to design transmission lines so that they have a uniformly distributed characteristic impedance from end-to-end. They also design the circuits at each end of a transmission line to terminate the transmission line with its characteristic impedance to avoid abrupt impedance changes at the transmission line ends.
When a driver sends a pulse edge of a digital signal to a receiver at a remote end of a transmission line, the pulse edge takes time to travel the length of a transmission line to the receiver because the driver needs time to change the charge on capacitance of the transmission line and to alter the magnetic fields in the transmission line inductance that oppose voltage changes on the transmission line. The velocity of signal propagation Vp of a transmission line having uniform distributed capacitance C and inductance L per unit length can be modeled by the expression:Vp=(1/LC)1/2  [2]
In many applications it is necessary to precisely control the signal path delay of a transmission line. For example an integrated circuit (IC) tester that tests an IC die on a semiconductor wafer sends test signals to the IC die under test (DUT) and samples output signals the DUTs produce in response to the test signals to determine whether the DUT is behaving as expected. The test and response signals travel over transmission lines between the tester and bond pads on the DUT surfaces that act as input/output terminals. The transmission lines include probes for accessing the DUT bond pads and various connectors and circuit board traces for linking the tester to the probes. Since the tester must carefully control the times at which test signal state changes arrive at the DUT and the times at which the tester must sample each response signal, the signal path delay of the transmission lines becomes increasingly large and problematic with increasing test and response signal frequencies. For example when the tester changes the state of two test signals at the same time, the state changes of those two test signals should arrive at separate terminals of a DUT acceptably close together in time. It helps to use transmission lines of similar design and length to convey the two signals from their sources in the tester to the DUT. However in high frequency testing environments, where signals timing must be accurate within the picosecond range, it is difficult to construct a set of transmission lines having sufficiently similar signal path delays.
One solution to the problem is to provide transmission lines having adjustable delays. As illustrated in FIG. 1 herein, U.S. Pat. No. 5,760,661 issued Jun. 2, 1998 to Marvin Cohn, describes a transmission line 10 including a trace 12 formed on a semiconductor substrate for conveying high frequency alternating current (AC) signals. As may be seen from equation [2] above, since the velocity of signal propagation Vp is inversely proportional to the square root of its capacitance, it is possible to control a transmission line's delay by adjusting its capacitance. A varactor diode acts like a capacitor having a capacitance that is a function of the voltage across it. Cohn teaches to connect varactor diodes 14 at various points along the trace 12 so that the capacitance of the varactor diodes increases the inherent capacitance of the trace. A pair of capacitors 20 at the ends of trace 12 block direct current (DC) signals but allow an AC input signal to pass over the trace. This prevents the common mode voltage of the AC signal from influencing the bias on diodes 14. A DC control voltage VBIAS delivered to trace 12 from an adjustable voltage source 15 through a resistor 28 controls the capacitance of the varactor diodes and therefore the delay of transmission line 10. Thus the delay of transmission line 10 can be adjusted by adjusting the DC control voltage VBIAS on trace 12. Although changing the capacitance of transmission line 10 also changes the transmission line's characteristic impedance, small reflections resulting from small mismatches in characteristic impedance can often be tolerated when small variations in signal path delay cannot.
Since the voltage of an AC signal having a larger peak-to-peak voltage can substantially increase or decrease the voltage across varactor diodes 14 depending on whether the AC signal is in its high or low voltage swing, the delay provided by transmission line 10 varies with the phase of the AC signal, and that kind of delay variation can distort the signal. Cohn resolves this problem by modifying transmission line 10 of FIG. 1 to form the transmission line 30 illustrated in FIG. 2. Here varactor diodes 32 of opposite polarity to that of varactor diodes 14 are also connected to trace 12. Any change in capacitance of varactor diodes 14 arising from variation in the AC signal voltage is offset by a substantially equal but opposite change in the capacitance of varactor diodes 32. Thus the total capacitance of the transmission line 30 is largely unaffected by the phase of the AC signal. by the cited patent are suitable for conveying high frequency sine wave signals, but they are not suitable for conveying other types of high frequency signals, such as binary digital signals. Versions of the transmission line employing isolation capacitors 20 would block or substantially distort such signals. Thus versions of the transmission line that do not employ blocking diodes add a DC offset to the voltage of the signal that may not be acceptable in many applications. Versions of the transmission line employing isolation capacitors to remove the DC offset would substantially distort high frequency signals that were not sine waves.
One drawback to the prior art transmission lines illustrated in FIGS. 1 and 2 is that they require relatively large numbers of discrete varactors to be distributed along the length of the transmission lines. Such transmission lines are expensive and difficult to fabricate.
What is needed is a transmission line having an adjustable signal path delay that is suitable for conveying all types of high frequency signals including but not limited to digital signals, analog signals, square waves signals, sine wave signals and combinations thereof, and which does not require the use of large numbers of expensive varactors.